Microscale and nanoscale patterns are used in a wide range of photonic and electronic devices and applications, including organic light emitting diodes (OLEDs), photovoltaic cells, thin-film electronic circuits, information displays, touch screens, wire-grid polarizers, metamaterial films, sensors, and many others. These devices may include components and sub-assemblies having very fine-scale circuitry and electrically conductive elements, typically fabricated using high resolution graphic arts techniques and/or semiconductor lithography.
High-resolution printing, including gravure, ink jet, etc., is a relatively inexpensive process and, limited by print resolution, is used to make the coarser patterns of these devices, typically ranging from hundreds of microns down to roughly ten microns in width. The electrical conductivity of traces made by “printed electronic” techniques, typically using metal particle or metal precursor type inks, are generally inferior to those made from bulk metals and often require thermal post-processing to achieve acceptable electrical conductivity. For producing patterns in the micron to submicron and nanometer regime, a newer process, nanoimprint lithography (NIL), is being used as an alternative to optical lithography, which is very expensive for nanometer-scale features. In NIL, a 3-dimensional (3D) master pattern (template or mold) is placed into contact with a layer of a liquid or deformable solid polymeric material, followed by the application of some amount of pressure, resulting in the polymeric material flowing into the template cavities to form the complementary structure. If the polymer material is in the form of a solid layer, heat or chemical treatment is used to soften the layer and allow it to flow into the template. After the material is solidified (either by reducing the temperature below the material's Tg or by radiation crosslinking), the template and polymer layer are separated, whereupon the polymer layer will have the (complementary) surface structure of the template.
NIL can also used to form a polymeric etch mask, essentially a stencil through whose openings material can be deposited or removed. In semiconductor lithography, such mask structures are commonly formed in a spin-coated photoresist material using optical exposure through a photomask, by direct laser or e-beam writing, or by the newer NIL process. On significant advantage of the NIL approach over optical lithography, particularly in forming very small (<100 nm) features, is that the expensive and complex optical exposure process is replaced by a much simpler mechanical imprinting process. In addition, as a parallel process, it is significantly faster than serial process of direct write lithography.
One limitation, however, of the NIL process for forming etch masks is that a very thin layer of residual polymeric material (known as the ‘residue’ or ‘scum’ layer) is left in the bottoms of the mask (i.e., closest to the substrate on which the mask is formed) after imprinting, and this layer must be removed prior to further processing (deposition or removal). Incomplete removal of the residual material from the mask will result in defects after subsequent steps: e.g., patches of missing metal after metallization in additive processing and stray patches of metal after etching in subtractive processing.
Residue removal (also known as ‘de-scumming’) is usually carried out by plasma (or reactive ion) etching, a vacuum process that is used to selectively remove unwanted organic or other material. Although ideally this is an anisotropic process, where material perpendicular to the source direction is removed at a faster rate than that in the parallel direction, this is not always the case, resulting in potentially significant unwanted etching of critical mask features, for example that results in widened mask openings that produces incorrect line widths.
In addition, the residue removal process has other drawbacks: 1) it requires expensive vacuum equipment with specialized gas handling and controls, 2) pump-down time to reach operation pressure adds to the processing time, 3) the etch process itself can be slow, also adding to the process time, 4) non-uniformities in the plasma field can cause non-uniform polymer removal and result in areas that are under-etched (areas of residue left intact) or over-etched (areas of mask polymer removed), and/or 5) the etch process can be detrimental to other elements of the structure (including by unwanted material removal, chemical interactions, re-deposition of etch by-products, hardening of the mask, etc.).
It is thus very desirable to be able to form mask layers that do not require plasma etch removal of the residue layer. Several processes have been developed and art well known to the art for doing this, including the use of semi-transparent or hybrid imprint masks and by modification of the surface-mask polymer wetting properties. Cheng and Gou (Xing Cheng and L Jay Guo, One-Step Lithography For Various Size Patterns With a Hybrid Mask-Mold, Journal Microelectronic Engineering, Vol. 71, No. 3-4, pg. 288-293, May 2004), Liao and Hsu (Wen-Chang Liao and Steve Lien-Chung Hsu, High Aspect Ratio Pattern Transfer in Imprint Lithography Using a Hybrid Mold, J. Vac. Sci. Technol. B 22, 2764, 2004) and Schift, et al. (Helmut Schift, Christian Spreu, Arne Schleunitz, Jens Gobrecht, Anna Klukowska, Freimut Reuther, and Gabi Gruetzner, Easy Mask-Mold Fabrication for Combined Nanoimprint and Photolithography, J. Vac. Sci. Technol. B 27, 2850, 2009) describe “hybrid mask-mold” processes (also known as Combined Nanoimprint and Photolithography, or CNP) in which certain portions of an imprint mask include thin film metal areas that block incident light and thereby prevent crosslinking of the underlying polymer material (what would otherwise be the residue), which is thus developable during subsequent processing. In another variant of this approach, Kao et al (Po-Ching Kao, Sheng-Yuan Chu, Chuan-Yi Zhan, Lien-Chung Hsu, Wen-Chang Liao, Fabrication of the Patterned Flexible OLEDs Using a Combined Roller Imprinting and Photolithography Method, 5th IEEE Conference on Nanotechnology, Volume 2, 693-695, 2005) used a hybrid mask in Hua et al. (Hua Tan, Andrew Gilbertson, and Stephen Y. Chou, Roller Nanoimprint Lithography, J. Vac. Sci. Technol. B 16, 3926, 1998) for a roller press nanoimprint lithography process. In non-CNP approach to “nonresidual layer imprinting”, Pina-Hernandez et al (Carlos Pina-Hernandez, Jin-Sung Kim, Peng-Fei Fu, and L. Jay Guo, Nonresidual Layer Imprinting and New Replication Capabilities Demonstrated for Fast Thermal Curable Polydimethysiloxanes, J. Vac. Sci. Technol. B 25 (6), November/December 2007) described a process in which thermal curable polydimethylsiloxane resists and fluorinated silane surface treatments were found in certain instances to form structures without residual layers.
However, each of these approaches has certain limitations and drawbacks which the subject technology, as will be described below, overcomes. In some approaches, such as with “nonresidue layer imprinting”, the requirement of a particular surfaces and surface treatments to reduce or eliminate the residue layer works under a restricted set of conditions, and further is not compatible with a broad range of materials, geometries or processes. In the case of the hybrid mask-molds, very thin metal films are used for the absorbing layers, which can result in light leakage into adjacent structures and partial exposure of the residue areas (Schift et al, FIG. 5). Furthermore, all of these processes use rigid substrates (glass, quartz) substrates which can be etched to produce the hybrid mask. Such masks are relatively fragile and expensive and are inflexible so as not to be suitable for commercial high-volume manufacturing, and in particular for roll-to-roll manufacturing.